1. The Field of the Invention
The field of the invention relates to interfacing computer devices with a digital network such as an Integrated Services Digital Network (ISDN) system. Additionally, the invention also relates to interfacing analog devices with the ISDN system, such as adapting analog modems to compatibly interface to the digital network. More particularly, the invention relates to the transmission of digital control information from a computer system directly to a digital network interface.
2. Present State of the Art
Through the recent decades, the majority of data transmitted between two locations occurred using analog transmission devices such as analog modems. An analog modem operates by modulating data and transmitting the modulated data as an analog signal across a transmission medium. Analog modems also receive modulated data as analog signals and perform the demodulation and conversion back to digital form. Most analog modems transmit and receive modulated data in analog form over the present public switched telephone network (PSTN) which employs a twisted pair of wires as the transmission medium. The twisted pair of wire transmission medium coupled with the switching and routing infrastructure of the communication system contribute to creating a bandwidth limited configuration. In communications where additional bandwidth was required, dedicated leased analog lines were employed to satiate increased bandwidth appetites. Such enhanced configurations typically were comprised of a plurality of twisted pair interconnection mediums.
A vast number of communication applications today require substantial amounts of data to be downloaded or transmitted through the communication medium. To accommodate a user's desire to have such substantial amounts of data dispatched within a reasonable amount of time, transmission services were developed capable of a higher band width. To provide compatibility with established communication infrastructure, telecommunication providers have begun to offer digital transmission services over communication medium previously facilitating only analog communications. Digital transmission services such as, for example, T1 services, E1 services, and Integrated Services Digital Network (ISDN). The majority of such digital transmission services employ the established twisted pair communication infrastructure medium, coupled to a digital network through a digital interface circuit. Such digital communication standards provide enhanced bandwidth capabilities for users. For example, a typical T1 connection employs a data rate of 1.544 MPS (Megabits per second) which may be comprised of a number of logical channels, such as 24, for distribution to a plurality of users. ISDN systems communication information and control signals through different channels which are multiplexed in a time divided fashion such as a time multiplex (TDM) signal. The TDM signal comprises two communication channels or B channels which are separately called B1 or B2 and a single control channel which is called a D channel. Each B channel is configured to facilitate transmission of 64 Kbps. while the one D channel operates at 16 Kbps. Specifications of ISDN, T1, and E1 services are described in CCITT (International Telegraph and Telephone Consultive Communication) recommendations.
FIG. 1 depicts a conventional interface with established components to an ISDN system. A conventional computer or terminal 100 provides the appropriate user interface, such as ISA, EISA, PCI, or PCMCIA, to an analog modem 102 and provides modulation and demodulation transformation of the data as described above. In prior art transmission configurations, analog modem 102 interfaces directly with the PSTN to provide modulated and demodulated analog data over the previously established network. To transform such analog modulated data for use on a digital network, a terminal adapter 106 is known in the prior art to transform the analog modulated data to digital format. Conventional transformations employed analog and digital converters to convert or quantize the modem modulated signal to digital form prior to transmitting such a signal over the digital network. Terminal adapters 106 having an analog input for receiving data from an analog modem 102 are known in the art. Terminal adapter 106 digitizes the modem signals and multiplexes the signals with other digital signals and transmits them accordingly throughout the digital network. Terminal adapter 106 interfaces with analog modem 102 over an established interface known as the R interface 104. Terminal adapter 106 further couples with a network termination (NT-1) 110 over a defined S/T interface 108. In turn, network termination 110 interfaces with ISDN system 114 over a defined U interface 112.
FIG. 2 depicts a more detailed block diagram configuration for interfacing a terminal 100 with ISDN system 114. Typically, an analog modem 102 is comprised of a microcontroller 120 for receiving digital data from terminal 100 directly and for controlling other operations and functions of analog modem 102. A DSP 122 receives digital data from microcontroller 120 and performs the modulation and demodulation functions associated with transforming digital data into analog modulated information. A CODEC 124 receives modulated data in digital form and performs the actual digital to analog and the reciprocal analog to digital transformations of such information. A data access arrangement (DAA) 126 provides the circuitry necessary for accessing or transmitting an analog signal according to the requirements of the communication network such as PSTN. This includes isolators, transformers, and other mechanism standards in the art allowing connection between systems having different electrical characteristics and requirements.
A terminal adapter 106 couples to analog modem 102 and receives data therefrom into an analog to digital converter 128. Analog to digital converter 128 transforms the analog data into digital form for processing by microcontroller 130. The transformed modulated digital data is then multiplexed into the digital communication B channels and alternatively, D channel previously discussed. An S/T interface 132 provides the appropriate interface required by network termination 110.
As shown, data originating in digital form from terminal 100 passes digitally to microcontroller 120 and thereafter is converted into analog form for transporting to terminal adapter 106. Upon receipt by terminal adapter 106, the originally digital signal that is now in modulated analog form is transformed again back to digital form by analog to digital converter 128. The digitized data retains the modulation information necessary for proper reverse conversion on the receiving end.
Some terminals 100, utilizing an analog modem 102 for transporting data, employ an ISDN system 114 as a transport network for distributing data with a recipient user. Such terminal devices 100 employing an analog modem 102 as their data transmission gateway, modulate the transmitted data prior to distributing such data in digitized format throughout the ISDN system. As such, when a destination terminal 100 receives digitized data, such digitized data must undergo a digital to analog transformation in the terminal adapter prior to being received by analog modem 102. Such additional data transformations may introduce distortion because of the multiple conversions which occur. Furthermore, each additional conversion which may introduce distortion, may result in a conversions errors and hence a loss of accuracy due to each conversion.
Additionally, terminal 100 must generate call setup information signals, typically DTMF tones, which are interpreted for use in establishing and routing the call. A typical analog modem 102 generates the DTMF tones for modulation over the PSTN. However, in a digital network, terminal adapter 106 which interfaces to ISDN system 114 must generate the call setup signaling by interpreting the call setup signaling that was once in digital format at terminal 100, but that has become modulated DTMF tones when received at terminal adapter 106. Analog to digital converter 128 in terminal adapter 106 then must quantize modulated DTMF tones which in turn are evaluated and interpreted by microcontroller 130 of terminal adapter 106. Such conversion from digital form at terminal 100 to analog modulated form and then back to a quantized form for consumption by terminal adapter 106 may introduce errors and create unnecessary inefficiencies for locally consumed call setup and control signals.
It would, therefore, be an advancement in the art to provide an apparatus for digitally conveying digital control signals and alert tones between an analog modem and a terminal adapter without the need to transition such control signals through an analog transformation.
Yet another advancement in the art would be to provide an apparatus for digitally conveying digital control signals and alert tones between a cellular modem and a terminal adapter without requiring the intermediate conversion of such signals into modulated analog signals.
A further advancement in the art would be to provide a method for digitally conveying control signals and alert tones between an analog modem and an ISDN system during a communication session.
A still further advancement in the art would be to provide a method for digitally conveying control signals and alert tones between a cellular modem and an ISDN system during a communication session.